Abstract: We examine the determinants of environmental regulatory activity (inspections and enforcement actions) for 1616 U.S. manufacturing plants in four large U.S. cities – Los Angeles, Boston, Columbus, and Houston – using data for 2000-2002. The main focus of our study is to examine whether or not regulators treat different segments of the population differently, by directing more regulatory activity at plants in rich, white neighborhoods and less in poor, minority neighborhoods, controlling for characteristics of the plant (size, age, and industry), and the plant’s past environmental performance. To date, tests of “Environmental Justice” hypotheses tend to focus on whether or not polluters are disproportionately likely to locate in neighborhoods with relatively high poor/minority populations, or on whether polluters located in those neighborhoods emit disproportionately high levels of pollution. Focusing instead on the allocation of enforcement activity across neighborhoods within each city allows us to shed light on a key mechanism through which discrepancies in pollution exposure across neighborhoods could arise and persist. Our results show relatively little statistically significant evidence that regulatory activity is less intense near disadvantaged demographic groups. We do find some suggestive coefficients - plants located in minority neighborhoods face less regulatory activity - but this effect is generally insignificant, and plants located in poor neighborhoods face (insignificantly) more regulatory activity. In contrast, we do find significant effects for plant characteristics and political variables, with plants that are larger and more energy-intensive, owned by single-plant firms, and located near politically active and liberal populations, facing more regulatory activity.